Along with cashmere romper suits and baby nail clippers, here is another thing for new parents to add to the "don't waste your money" list: those baby DVDs and videos which claim to give infants a boost in learning language.

According to scientists in the US, watching "educational" DVDs with titles such as Baby Einstein and Brainy Baby does not help a baby's linguistic development. What's more, overuse may slow down infants aged eight to 16 months when it comes to acquiring vocabulary.

A study, published today in the Journal of Paediatrics, showed that for every hour per day spent watching baby DVDs and videos, infants understood an average of six to eight fewer words than infants who did not watch them.

Academics from the University of Washington and the Seattle Children's Hospital Research Institute conducted random interviews with more than 1,000 families in Minnesota and Washington with a child born in the previous two years. Researchers asked parents with children aged eight to 16 months how many words from a list of 90 their child understood.

Words included choo choo, mommy and nose. Those with 17-24-month-old children were asked how many words on a similar list they had heard their child use. Typical words were truck, cookie and balloon.

All parents were asked what TV, DVD and videos their children watched, as well as how often they were read to.

Andrew Meltzoff, co-director of the University of Washington's Institute for Learning and Brain Sciences and co-author of the study, said: "The results surprised us, but they make sense. There are only a fixed number of hours that young babies are awake and alert. If the 'alert time' is spent in front of DVDs and TV instead of with people speaking in 'parentese' - that melodic speech we use with little ones - the babies are not getting the same linguistic experience." The researchers believe the content of baby DVDs and videos is different from the other types of programming because it tends to have little dialogue and shows linguistically indescribable images such as a lava lamp. By contrast, children's educational programs are crafted to meet developmental needs of pre-school children.

Dimitri Chirstakis, a paediatrics researcher at Seattle Children's Hospital Research Institute and co-author of the study, said: "I believe the onus is on the manufacturers to prove their claims that watching these programs can positively impact children's cognitive development."